1. Field of the Invention
This invention relates to a method for enhancing the acid activity of a porous crystalline aluminosilicate zeolite, including high silica-containing porous crystalline zeolite materials, which comprises contacting the zeolite with a solution comprising a hydrate of trivalent metal fluoride in aqueous media under conditions sufficient to cause incorporation of the trivalent metal in the framework of the zeolite. The resulting material can be contacted with warm aqueous solution of an ammonium salt, and thereafter calcined to produce a zeolite composition exhibiting enhanced Bronsted acidity.
2. Description of Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejection those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework SiO4 and AlO4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic aluminosilicates. These aluminosilicates have come to be designated by convenient symbols, as illustrated by zeolite ZSM-5 (U.S. Pat. No. 3,702,886).
High silica-containing zeolites are well known in the art and it is generally accepted that the ion exchange capacity of the crystalline zeolite is directly dependent on its aluminum content. Thus, for example, the more aluminum there is in a crystalline structure, the more cations are required to balance the electronegativity thereof, and when such cations are of the acidic type such as hydrogen, they impart tremendous catalytic activity to the crystalline material. On the other hand, high silica-containing zeolites having little or substantially no aluminum, have many important properties and characteristics and a high degree of structural stability such that they have become candidates for use in various processes including catalytic processes. Materials of this type are known in the art and include high silica-containing aluminosilicates such as ZSM-5, ZSM-11 (U.S. Pat. No. 3,709,979), and ZSM-12 (U.S. Pat. No. 3,832,449) to mention a few.
The silica-to-alumina ratio of a given zeolite is often variable; for example, zeolite X (U.S. Pat. No. 2,882,244) can be synthesized with a silica-to-alumina ratio of from 2 to 3; zeolite Y (U.S. Pat. No. 3,130,007) from 3 to about 6. In some zeolites, the upper limit of silica-to-alumina ratio is virtually unbounded. Zeolite ZSM-5 is one such material wherein the silica-to-alumina ratio is at least 5. U.S. Pat. No. 3,941,871 discloses a crystalline metal organosilicate essentially free of aluminum and exhibiting an x-ray diffraction pattern characteristic of ZSM-5 type aluminosilicate. U.S. Pat. Nos. 4,061 724; 4 073 865 and 4,104,294 describe microporous crystalline silicas or organo silicates wherein the aluminum content present is at impurity levels.
Because of the extremely low aluminum content of these high silica-containing zeolites, their ion exchange capacity is not as great as materials with a higher aluminum content. Therefore, when these materials are contacted with an acidic solution and thereafter are processed in a conventional manner, they are not as catalytically active as their higher aluminum-containing counterparts.
Methods for enhancing the catalytic activity of zeolitic materials are well known, including treatments involving fluorine. U.S. Pat. No. 4,444,902 relates to a process for enhancing acidic activity of a highly siliceous zeolite by contact with aluminum fluoride followed by ammonium exchange and calcination. U.S. Pat. No. 4,427,787 teaches zeolite activation by contacting an alumina-composited zeolite with hydrogen fluoride. Ammoniacal aluminum fluoride is taught as a reagent for zeolite activation in U.S. Pat. No. 4,427,788. U.S. Pat. No. 4,427,790 treats enhancing zeolite activity by treatment with a compound of the formula L.sub.(n-m) [MF.sub.n ] wherein L is an organic or inorganic ionic moiety [MF.sub.n ] is a fluoroanion moiety wherein M is a Group VB, VIB, VIIB, VIII, IIIA, IVA or VB element, n is the coordination number of M, m is the valence of M and e is the charge associated with L. U.S. Pat. No. 4,444,902 discloses the use of metal salts to activate zeolites such as Al(NO).sub.3 which is followed by addition of a source of fluoride ions. U.S. Pat. Nos. 4,477,582 and 4,559,131 teach reactivation of steamed catalysts by contact with aqueous metal salt solution, e.g. halide. U.S. Pat. Nos. 3,354,078, 3,644,220 and 4,576,805 relate to treating crystalline aluminosilicates with volatile metal halides.
The novel process of this invention permits the preparation of certain high silica-containing materials which have all the desirable properties inherently possessed by such high silica materials and, yet, have an acid activity which heretofore has only been possible to be achieved by materials having a higher aluminum content in their "as synthesized" form. It further permits valuable activation of crystalline zeolites having much lower silica-to-alumina mole ratios.